This invention relates to an improved electrostatic recording sheet and to a method for making the same, and more especially, to an improved method for making an electrostatic recording sheet having superior recording properties by coating directly on an electroconductive base sheet a coating composition consisting essentially of an aqueous dispersion of polymers to form a dielectric layer.
The electrostatic recording system has recently been widely applied to a facsimile or a computer input-output system suitable for high speed recording in a high speed telecommunication system, a high speed graphic reproduction system, etc. The electrostatic recording material as a recording medium basically comprises a highly dielectric layer, which serves as an electric-charge-retentive layer, and an electroconductive base sheet which supports the dielectric layer. Electrostatic images of electrical signals formed on the dielectric layer are made visible with a developer comprising a toner and a carrier which has a polarity opposite to the polarity of the electrostatic image charge, and fixed as permanent visible images by further treatment.
It is known to use high molecular substances such as polystyrene, polyacrylates, polyvinylidene chloride, polyvinyl acetate, polyvinyl acetal, nitrocellulose and silicone resin for forming such a dielectric layer. Usually these high molecular substances are applied onto a base sheet in the form of solutions prepared by being dissolved into an organic solvent such as acetone, toluene, benzene, methylethyl ketone and the inflammability Use of such organic solvents is disadvantageous because of their inflamability and poisonous characters when vaporized in addition to the fact that they are usually expensive.
In order to avoid these disadvantages with organic solutions some attempts have been made to use aqueous coating compositions to form the dielectric layer on a base sheet. In this case, however, the treatment for reducing an electric resistivity of the base sheet is usually carried out by coating or size press impregnating conventional highly hydrophilic electroconductive agents such as hygroscopic inorganic salts, hygroscopic polyalcohols, activators of quaternary ammonium polyelectrolytes and the like, consequently the electroconductive agent on the surface of the base sheet or in the base sheet dissolves and migrates into the aqueous coating composition to result in degrading the electrostatic characteristics of the dielectric layer. Thus, using such aqueous compositions involves fatal defects such as deterioration in its charging retentive and dielectric properties.
Another attempt has also been made to form a barrier layer between an electroconductive layer and a dielectric layer to prevent migration of the electroconductive agent into the dielectric layer. However, this involves an economical disadvantage for the additional step therefor and a functional defect that the recording sensitivity is lowered due to an increase in the thickness of the dielectric layer.
It is disclosed in U.S. Pat. No. 3,861,954 to prevent penetrating of the electroconductive agent into a base sheet by using a dense paper sheet having a bulk porosity of less than 200 Sheffield units as measured by a Sheffield Porosimeter with a 11/2 inch orifice and air at 11/2 psi. as base sheet for forming an electrostatic recording material with an aqueous composition, to form relatively thin layer of an electroconductive agent on the dense paper base sheet and then to form a dielectric layer on said thin layer of an electroconductive agent. This method, because of using a special dense paper base sheet having the low Sheffield porosity as base sheet, is advantageous in that a uniform layer of an electroconductive agent can be formed, but preparation of the base sheet is troublesome because it is necessary to blend pulps in a certain special manner and to carry out high-level mechanical refining of used pulps to obtain such base sheet. Further, increasing the density of a base sheet inevitably involves deteriorating the dimensional stability to make the base sheet curly.
Additionally, in the method disclosed in U.S. Pat. No. 3,861,954 with the amount of the electroconductive agent in excess of 0.5 g/m.sup.2, the obtained electroconductive layer exhibits a poor sizing degree because of high hydrophilic property of the electroconductive agent, and the aqueous coating composition for forming the dielectric recording layer penetrates instantaneously into the layer of the electroconductive agent to result in forming an ununiform dielectric recording layer. Such a recording sheet with an ununiform dielectric recording layer is liable to get an increased electric capacity, a reduced charging potential and irregular charging. Accordingly, such a recording sheet cannot get uniform density of recorded images.
On the other hand, with the electroconductive agent below 0.5 g/m.sup.2, only a minimum amount of the electroconductive agent penetrates into the dielectric layer so that the insulating properties of the dielectric layer is kept in relatively good condition. However, the amount of the electroconductive agent used is so small that the conductivity of the electroconductive layer is deteriorated under low humidity conditions and no satisfactory images can be produced on the recording sheet, although relatively good imges can be produced under mild as well as high humidity.
The principal object of the invention is to provide an improved method for making an electrostatic recording sheet wherein an aqueous coating composition of highly insulating synthetic resin can be directly applied onto the base sheet which has been subjected to an electroconductive treatment in such a manner that the above mentioned various disadvantages with the prior art can be overcome.
Other objects and advantages of the invention will become apparent from the following description.